A bifunctional electrode structure for a unitized proton exchange membrane regenerative fuel cell has been developed. The electrode has only a thin catalyst layer; it reduces the loading of the catalyst to 0.4 mg cm(-2), and minimizes mass transport and ohmic limitations. A satisfactory performance of a unitized proton exchange membrane fuel cell is achieved with this electrode structure. 50 wt.% Pt + 50 wt.% IrO2 is a good bifunctional catalyst for the oxygen electrode. Examination of this catalyst by transmission electron microscopy and of the electrode by scanning electron microscopy is reported. (C) 1999 Elsevier Science S.A. All rights reserved.A bifunctional electrode structure for a unitized proton exchange membrane regenerative fuel cell has been developed. The electrode has only a thin catalyst layer; it reduces the loading of the catalyst to 0.4 mg cm(-2), and minimizes mass transport and ohmic limitations. A satisfactory performance of a unitized proton exchange membrane fuel cell is achieved with this electrode structure. 50 wt.% Pt + 50 wt.% IrO2 is a good bifunctional catalyst for the oxygen electrode. Examination of this catalyst by transmission electron microscopy and of the electrode by scanning electron microscopy is reported. (C) 1999 Elsevier Science S.A. All rights reserved.